Question

Strong base is dissolved in 675 mL of 0.200 M weak acid (Ka=3.87×10−5) to make a buffer with a pH of 4.04. Assume that the volume remains constant when the base is added. HA(aq)+OH−(aq)⟶H2O(l)+A−(aq) Calculate the pKa value of the acid, and determine the number of moles of acid initially present. When the reaction is complete, what is the concentration ratio of conjugate base to acid? [A−][HA]= How many moles of strong base were initially added?

Answer 1

To calculate the pKa value of the weak acid, use the Henderson-Hasselbalch equation, and to determine the moles of acid initially present, use the concentration-volume relationship. The concentration ratio of the conjugate base to acid can be obtained using the equation [A-]/[HA]. The number of moles of strong base initially added is equal to the number of moles of weak acid initially present.

Step-by-step explanation:

The pKa value of an acid can be calculated using the Henderson-Hasselbalch equation:

pKa = pH + log([A-]/[HA])

Given that the pH of the buffer is 4.04, we can rearrange the equation to solve for pKa:

pKa = 4.04 + log([A-]/[HA])

To determine the number of moles of acid initially present, we can use the equation:

moles = concentration x volume

Substituting the values given in the question:

moles = 0.200 M x 0.675 L

The concentration ratio of the conjugate base to acid can be calculated using the equation:

[A-]/[HA]

The number of moles of strong base initially added can be determined by the stoichiometry of the reaction:

1 mol strong base neutralizes 1 mol weak acid

So the number of moles of strong base initially added is equal to the number of moles of weak acid initially present.